In recent years, there has been increased use by surgeons of computer assisted surgical navigation devices to assist them in determining the location of their surgical instruments relative to a patient's anatomy. Many of these surgical navigation systems and devices utilize small “point sources” of light and sensors for these light sources to determine the spatial position of a surgical tool relative to a patient's anatomy. The light sources historically have been light emitting diodes (LED's). A typical surgical tool includes multiple LED's positioned such that these LED's do not lie in a single plane. The determination of the location of these LED's for a properly calibrated tool can determine both the position of that tool and also the orientation of that tool within a three dimensional surgical space or volume.
Typical surgical navigation systems utilize multiple separately spaced apart detectors and associated computer software to determine the position of each of the LED's on a surgical device or instrument. In these systems, the light sources are small relative to the distance and volume and therefore are considered to be point sources of light. Typical LED's emit infrared light; however, light of other wavelengths, including light in the visible spectrum, can be used. These systems determine the relative angle of each light source entering an individual detector and the system combines the angle from each detector to calculate the three dimensional x, y, z coordinates for a particular light source. From the position of the LED's on a surgical instrument, the system can calculate the exact location of a properly calibrated instrument.
Typical detectors for these light or radiation sources may be position sensitive detectors (PSD), linear photo diode arrays or linear charged coupled devices (CCD). In the case of the CCD, these typical systems include a CCD array that is one pixel wide and a thousand or more pixels long, so that a significant volume can be detected. Typically the light from the point source passes through a lens or narrow slit so as to focus a thin line of light onto the CCD that in turn will illuminate a small number of contiguous pixels. Depending upon the size of the pixels in the CCD and the ability of the system to focus the light, the position of the light on that detector is typically assumed to be either the center point of the pixels that have been illuminated or the pixel with the highest light intensity.
As surgical navigation and other position detection systems become more accurate, the accuracy of the lens or the slit that focuses the light from the point source onto the position sensing device becomes a limiting factor for system accuracy.